MEDFORD — The city’s School Committee, scrambling to minimize the fallout of a scandal involving the discovery of an ammunition magazine at a middle school in December, has canceled school in the district on Monday in order to hold an emergency professional development day.

The decision came during a heated and long committee meeting a Medford High School Thursday night, which more than 200 people attended.

Mayor Stephanie M. Burke had announced at a press conference earlier Thursday that McGlynn Middle School principal Jake Edwards had been put on paid administrative leave as the city continues to investigate why the discovery of the magazine in the school auditorium on Dec. 29 was not promptly shared with her office and Medford police.

“I am both professionally and personally astounded that I was not informed immediately of the incident,’’ Burke said at the press conference.

At the School Committee meeting, residents criticized how School Superintendent Roy E. Belson handled the discovery of the magazine.

“I don’t know where Mr. Belson’s head was in that he thinks it was his sole responsibility to decide what should have been done,’’ said Suzanne Henson.

“The fact of the matter is that these people don’t trust you,’’ resident Doug Nagengast said to Belson.

Belson, who in November said he plans to retire at the end of the school year, acknowledged that he should have notified authorities sooner.

“I made a mistake, and that will never happen again as long as I’m in this role,’’ he said.

Police Chief Leo Sacco said there were “at least three, maybe four bullets’’ in the magazine, which was found during school vacation. Burke said she did not learn of the discovery until last Friday. Police were also kept in the dark until then, the mayor said.

Burke said the way the episode was handled was “unacceptable’’ and “put our entire community at grave risk.’’

“I am deeply concerned about the way this matter was handled by the school administration, the lack of attention to the potential threat the situation imposed, and the lack of notice and communication with police personnel, as well as my office,’’ she said.

In a statement posted on the school system’s website, Belson wrote that a cleaning company found the magazine under a seat in the auditorium and turned it over to the school custodian, who locked it up in the principal’s office. Belson wrote that the magazine may have been thrown out the following day. “Edwards was in his office on December 30, 2017, for the purpose of cleaning his office, and he claims he threw several items away that could have included the clip,’’ Belson wrote.

Burke said that Martha Coakley, the former attorney general, will investigate.

Burke also said Medford police and other law enforcement agencies conducted a sweep of the city’s schools, with dogs, on Thursday. The police chief said that by mid-morning no contraband had been found.

Sacco said it was unclear how the ammunition magazine got to the school.

He said it’s possible it was inadvertently left behind by a lawful gun owner. “We hope it’s something as simple as that,’’ he said. If that is the case, “we ask that person to come forward.’’

Emily Sweeney can be reached at esweeney@globe.com. Follow her on Twitter @emilysweeney. Alana Levene can be reached at alana.levene@globe.com